Compact membrane separation cells were developed in the 1960's based on bundling together fine strands of continuously hollow fibers. The design of this type of a separation cell shown in U.S. Pat. No. 3,228,876 employs from between 10 to one million or more hollow fiber strands having their end portions secured in tube sheet with the central portion of the fiber bundle exposed within the cell to facilitate membrane transport between the bores of the fibers and the interior space of the cell. Permeation separations are carried out with these units by feeding liquid through the multiple channels formed by the fiber bores with permeation occurring into the interior of the cell as the liquid simultaneously flows through the fiber bundle; or a feed liquid (or feed gas) may be contacted with the outer surfaces of the fibers, while a receiving liquid is passed through the bores of the fiber bundle. The permeable separable component is thus removed from the separation cell dispersed in the receiving liquid.
These separation cells are designed for relatively easy fabrication, and advantageously permit a maximum exposure of membrane surface area to the feed stream. The referenced patent claims as much as 10,000 square feet (10,000 ft.sup.2) of membrane surface area may be exposed for permeation transfer per cubic foot of volume occupied.
The small tube or fiber bore sizes necessarily used in these units to maximize membrane surface area, however, produce detrimental laminar flow conditions in which an effectively stagnant layer of liquid film develops that is next to the inner wall of the fiber. There is little or no convective mixing in this layer, and thus the permeable component moves across the stagnant boundary layer only by the relatively slow diffusion process. A resistance to transport can thus develop which retards the rate of mass transport of permeable component across the membrane apart from the resistance of the membrane itself. This effect is generally described as concentration polarization.
With efficient membranes, concentration polarization frequently becomes the rate limiting step in membrane transport. In such cases, improvements in membrane performance, such as by using thinner active membranes, leads to little or no gain in improving the efficiency of the separation cell.
Skilled practitioners concerned with improving the separation cell design of U.S. Pat. No. 3,228,876 to reduce concentration polarization have generally proposed a mechanical solution similar to the mechanical turbulence promoters having known effectiveness for membrane separations broadly. For example, external deformations of the fiber bundle or individual fibers has been frequently proposed (e.g., U.S. Pat. Nos. 3,963,622; 4,219,426 and 3,989,626). These designs, however, are not known to be commercially used, and are believed to produce only nominal improvements.
Packing the bore of small tubes with, e.g., spheres, has been recently developed for analytical chromatography and has been shown to be effective in reducing concentration polarization (Anal. Chem., Vol. 54, No. 7, pp. 1206-1208, 1982). However, the fabrication difficulty imposed by packing the individual fibers and the much higher back pressures produced make this technology impractical for large scale separations of process fluids.